Structure of Materials : Bonding in Materials

Bonding in Materials

The structure of materials affects the properties, and the process can engineer structure. We looked at the structure has various length scales; they are macro, micro, nano, electronic and atomic, etc. So, the macrostructure typically looked at by the naked eye. For micro, nano, or atomic structure, you go from optical to SEM to a TEM; this is the typical progression as you go from micro to nanostructure.

Then for the atomic or electronic structure, you have to do simulations typically. We discussed the structure of materials from the materials tetrahedron point of view because the structure is integrated intricately related to properties, processes, and applications. So, earlier, we classified materials in four categories, the first category is metals and alloys, second was ceramics and glasses, the third category was polymers and elastomers, and the fourth category was hybrids or composites.

 

As we know, metals are strong, ductile, and tough. However, they have poor corrosion resistance, they have high thermal conductivity, ceramics, on the other hand, are brittle, but they are very strong. However, they have low electrical and thermal conductivity, by and large, polymers, on the other hand, are soft, light, they can be stretched too long distances.

They are also tough, and they are also very corrosion resistant, but they are not very good for high-temperature applications. Composites, on the other hand, are manufactured by mixing two contrasting materials to leverage the advantage of both the different classes of materials. So far, we discussed one way of classifying the materials, and now we are going to discuss the material classification based on atomic bonding.

For example, metals and alloys are bonded via metallic bonds. Ionic bonds or partially covalent bonds bond ceramics and glasses. For example, sodium chloride would be highly ionic bonding; bonding in silicon carbide and zinc oxide will have partially ionic and covalent in bonding. Polymers, on the other hand, they have a mixture of covalent and secondary bonding.

Moreover, it is the nature of these bonds, which is very crucial in imparting the properties which these materials have. Metals have high electrical conductivity, high thermal conductivity, malleability, or ductility because of metallic bonding. Ceramics are strong, and they have low electrical, thermal conductivity, and they have a low coefficient of thermal expansion because ionic bonds or covalent bonds bond them.

Polymers, on the other hand, are soft, low strength because they are primarily, there is a mix of covalent and secondary bonding, and the secondary bonding plays an important role in determining the properties. So, we will briefly look at first the bonding aspects of various materials, before we get on to the atomic structure of materials. So, let us begin with what we call as bonding; it is not a full course on bonding; it is just the primer on bonding.

We know that you have an atomic structure. So, depending upon the atomic number, you can have a nucleus, and this nucleus has protons and neutrons and surrounding various electrons. So, you can have to depend upon the atomic number, and you have 1s2, 2s2, 2p6, and so on. If Z is equal to 10, you will make a structure 1s2, 2s2, 2p6, and as you go higher, you can keep building the atoms.

An atomic mass of material of element is equal to Z, which is the atomic number plus N, which is the number of neutrons. These are two things we know about the atoms because everything is made of atoms. So, the atomic structure has to be understood well before you go into bonding.

What are the characteristics of electrons in atoms? The energy levels of electrons are you can say discrete or quantized. So, there are specific energy levels, which electrons occupy, and the tendency is to occupy the lowest energy state first. As the lowest energy states are filled, then the higher energy states are filled. So, for example, you can have n, n is equal to 1, this is the one, and as you go to E1, and if n is equal to 2, you go to E2, and as you go to n is equal to 3, you go to E3, and this is basically increasing energy.

So, completely filled energy levels, like Helium, Neon, Xenon, and Krypton, are all these have completely filled energy levels called inert gases. The electronic shells are not completely filled, for example, if you look at Iron have an atomic number of 26, it makes 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, and what do you have then? 3d6 and d-orbital can have 10 electrons, but it has only 6 electrons, so it is partially filled.

Now, depending upon the presence of Iron atoms, there is a tendency to give or take the electrons, which is called electronegativity electro positivity. This can determine what kind of bonding they will have, or sometimes what happens is that they do not necessarily have to give away the electron or to take the electron they can share the electrons. Depending upon how the electrons are configured or shared between the atoms, they undergo certain kinds of bonding.

So, as we can see that except in an inert gas, all elements have unfilled electronic orbitals, and as a result, they are unstable, which means to be stable, they have to have a stable configuration. So, what should they do? They should do something with these electrons, which incomplete configuration of electrons in the outer shells have to be taken away or they have to be shared something has to happen, and that is why these are called valence electrons.

These valence electrons are the outer shell electrons, which are not equal to the number of the total number of electrons in that particular orbit can have. However, if you look at the periodic table, you have various columns in the periodic table.

So, we can have I A, II-A, III B, IV B, V B, VI B, and VII B, so this goes to 7, and after that, then you go to 1B and so on. So, on the extreme right, you have inert gases. On the left, you have elements which are called as electropositive, on the right, just before the inert elements you have electronegative elements. Moreover, what is this specific thing about electropositive elements? Electro positive elements tend to donate or to give away their extra electrons, which are the incomplete shell incomplete number of the extra electrons, which are lying in that shell, and these electronegative elements tend to accept.

Elements like sodium, potassium, magnesium, calcium all the elements on this side they have they are electropositive, they tend to give away the electrons. On the other hand, things like chlorine, fluorine, bromine, iodine tend to accept electrons. Similarly, Oxygen, Sulfur, and so on tends to accept electrons. So, on one side, you have atoms that tend to donate electrons, and on the other side, you have atoms that are a tendency to accept the electrons. In the middle, you have atoms right up to column 3, the boron aluminum you mostly have elements, which will tend to give away the electron.

So, one side, you have electropositive elements, and another side, you have electronegative elements. And when you mix these elements, you form bonds. Because one tends to give away another has a tendency to take the electrons, and that is where the bonds are formed. So, typically, electronegativity ranges.

 So, this is determined by that the nature of electro positivity or electronegativity. Electronegativity is determined by a value called electronegativity, and which ranges from 0.7 to 4. So, 0.7 will be electropositive or less electronegative, and four will be highly electronegative. For example, Lithium, this is denoted by a parameter called χ. So, for Lithium, this value is typically 1, for sodium, it is about 0.9, for Potassium it is about 0.8. If you look at column 2, Magnesium is about 1.3, and Calcium is about 0.13, if you go a little further, then Titanium has a value of about 1.5, Zirconium has a value of about 1.3. If you go further right, chromium has a value of 1.7, Manganese has a value of 1.6, Iron has a value of 1.8, and cobalt has a value of 1.9, Copper has a value of 1.9.

You can see that most of these elements have electronegativity, which is slightly on the lower side is starting from Lithium, which starts at 1, it goes right up to about two for most metals. So, they are in some sense, like strongly electropositive or moderately electropositive. So, let me define here, this is strongly electropositive. This would be strongly electronegative. So, these are typical metals. Now if I go to other classes another side of the periodic table, you start from fluorine, fluorine has a value of 4.

Chlorine has a value of 3; Iodine has a value of 2.7, Oxygen has a value of 3.5, Sulfur has a value of 2.5, Nitrogen has a value of 3, Phosphorous has a value of 2.2, Carbon has a value of 2.5. These are strongly electronegative, and these elements make compounds that is why you see in nature many things appear as carbides, nitrides, oxides, sulfides, iodides, chlorides, because these elements are ready to react with other elements to take their electrons. So, this is the basis of distinguishing between the items.

 Based on these parameters, you have an excess number of electrons and the outer shells, or there is unstable configuration, which allows metals to do something with these extra electrons so, that it becomes a stable configuration, they tend to form bonds. Moreover, these outer shell electrons which are not sufficient number, they are called valence electrons and how they react how they how these different elements combine it depends upon the differences in the electronegative.

So, if you have a heterogeneous material, they will have to they form bonds because there is a tendency to accept and take they will give the electrons, but if you do not have heterogeneous compound for example, say Iron or say only Copper or say only aluminum, in that case there is some other mechanism of a doing something with those electrons. So, this is where we come to bonding.

Bonding is determined by how electrons are valence electrons. How are valence electrons handled by atoms when they are ensembled together. So, you have two cases you can have heterogeneous mixtures more than one element, and you can have a single element. So, for example, this would be mean you can say compounds not heterogeneous, but I would say compound.

For example, things like sodium chloride, you think oxide and so on, and here it would be anything single like Iron, Cobalt, Nickel, Aluminum. So, let us see what different kinds of bonds are. Now before we go into bonding, we need to understand the fundamentals of what happens when you put the atoms together, and that is that you can understand by understanding the interatomic.

So, this I am covering very fast because this is not the basically basis of this course. But you must know this, that is why there is a recap of this particular interatomic forces in. So, when you bring atoms to atoms together, these atoms are located at a distance r, which is the equilibrium distance. And why is this equilibrium distance, what is the significance of this equilibrium distance? Because when you bring these atoms together, the forces between the atoms, is F, and there are two kinds of forces first is the repulsive force, and second is the attractive force.

So, this is let us say Fr this is Fa, and this is the distance r. The stable configuration is the configuration where the net force is equal to 0. So, this is the stable configuration at a distance r0 at which the force is equal to 0. Correspondingly you can plot what we call as the energy the potential energy; let us say E potential energy. This potential energy at this particular point it should be minimum right. So, if you plot the potential energy, the distance r0 at which you have potential energy which is defined by or E0 or w0 depending upon how you, in this case, we can say just e naught and this e naught is called as the bond energy.

The distance and the energy corresponding to the equilibrium distance between the atoms are called the bond energy. So, this potential energy can be given as the sum of the repulsive term and the sum of the attractive term. At some r value, the E value is minimized, which gives you the bond energy. So, what we will do in the next lecture is, we look at different kinds of bonds and our materials, and then we will move on to the structure of materials.

For Next Lecture Click below

 Structure of Materials : Bonding in Materials

Structure of Materials : Correlation between bond and physical properties

 

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